The invention pertains to network echo cancellation. The invention is particularly suitable for use for hybrid echo cancellation at a two wire to four wire interface in a communications network such as a telephone network.
In communications networks, it is possible for an echo of an upstream signal to be coupled onto a downstream signal (directions arbitrary), which echo either corrupts data or decreases the quality of data (e.g., increased noise). For instance, in telephone communications networks, the individual customers usually couple into the main telephone network through a two wire, analog connection in which transmissions in both directions are carried on the same pair of wires (tip and ring). However, the central portion of the network typically is a four wire, digital system in which communications in the upstream and downstream directions are carried on separate wire pairs (i.e., two wires in the receive direction and two wires in the transmit direction).
The interface between the two wire and the four wire portions of the network are a source of echo. The echos typically result because the impedance of the 2-wire facility is imperfectly balanced in the 4-to-2 wire junction causing the incoming signal to be partially reflected over an outgoing path to the source of incoming signals.
The circuitry interfacing the two wire and four wire portions of the network is commonly termed a line interface card. The standard components of a line interface card are well known to persons of skill in the related arts. One of those components is a hybrid cancellation circuit, the function of which is to cancel echo signals in order to improve the quality of service. In short, a hybrid cancellation circuit generates an attenuated version of the original receive signal (that is the source of the echo) and subtracts it from the signal lines that carry the transmit signal onto which the echo has been imposed. These hybrid cancellation circuits utilize an adaptive filter to operate on a supplied signal in a prescribed manner such that a desired output signal is generated.
Typically, adaptive filters generate a transfer function according to an algorithm that includes updating the transfer function characteristic in response to an error signal. In this manner, the filter characteristic is optimized to produce a desired result.
When used in an echo cancellation circuit, an adaptive filter is used to generate an echo path estimate that is updated in response to an error signal. Adaptive echo cancelers have been employed to mitigate the echoes by adjusting the transfer function (impulse response) characteristic of an adaptive filter to generate an estimate of the reflected signal or echo and, then, subtracting it from the outgoing signal. The filter impulse response characteristic and, hence, the echo estimate is updated in response to the outgoing signal for more closely approximating the echo to be canceled.
Various designs and algorithms for adaptive filters for echo cancellation are well known. Although prior art arrangements of adaptive filters perform satisfactorily in some applications, often it is impossible to simultaneously achieve both sufficiently fast response to changing echo paths and sufficiently high steady-state estimation quality. Consequently a continuing need is to achieve more rapid response to changing conditions while at the same time maintaining adequate steady-state estimation quality.
FIG. 1 is a block diagram illustrating an exemplary network echo cancellation circuit 10. Input line 12 carries the far-end signal x(n) (i.e., the receive direction signal relative to the transceiver having the echo cancellation circuit) Input line 14 carries the near-end signal v(n) as well as background noise w(n). The far-end signal x(n) is the source of potential echo.
Block 16 represents the true echo path h which is coupled into the near-end signal on line 14 as represented by summation node 18. In accordance with an echo cancellation scheme, the far-end signal x(n) is also input into an echo cancellation circuit 20 which generates an estimated echo path h(n). The estimated signal path h(n) is generated using a digital adaptive filter algorithm as illustrated by block 124. The estimated echo signal is then subtracted at subtraction circuit 26 from the signal path y(n), which includes the true echo h, the near-end signal v(n), and the background noise w(n).
Other circumstances in which network echo cancellation may be necessary or at least desirable are numerous and would be well known to persons of skill in the related arts.
Even further, acoustic echo cancellation systems are known in the prior art such as in connection with audio teleconferencing equipment. Particularly, when using a speaker phone, there is a path between the telephone speaker and the telephone microphone through which an echo can be introduced. Particularly, the microphone in a room picks up the sound created in the room. Part of the sound created in the room includes the sound coming from the opposite end of the telephone call that emanates from the telephone speaker. This creates a feedback loop that can result in distortion and, in the worst cases, howling instability.
Many digital adaptive algorithms for echo cancellation are known in the prior art, including Least Mean Squares (LMS) algorithms, Normalized Least Mean Squares (NLMS) algorithms, and Proportionate Normalized Least Mean Squares (PNLMS) among others.
U.S. Pat. No. 5,951,626 issued to Duttweiler and assigned to the same assignee as the present application discloses a PNLMS algorithm that has a very fast initial convergence and tracking of the true echo path when the echo path is sparse, i.e., relatively few of the coefficients are non-zero, compared to other methods such as NLMS. However, when the echo path is dispersive, PNLMS converges much more slowly than NLMS. Simulations support the conclusion that the full benefits of PNLMS in terms of fast convergence and tracking of the true echo path are achieved only when the impulse response is close to a delta function.
Accordingly, it is an object of the present invention to provide an improved adaptive filter.
It is another object of the present invention to provide and improved echo cancellation method and apparatus.
It is a further object of the present invention to provide an algorithm particularly adapted for echo cancellation that has fast convergence and echo path tracking over a broad range of echo paths.
The invention is a method and apparatus for performing adaptive filtering, and particularly echo cancellation, utilizing an efficient and effective adaptive algorithm. The invention is particularly useful in connection with network echo cancellation but is more broadly applicable to any situation where an adaptive estimate of a signal must be generated in real-time.
The invention includes an improved proportionate normalized least mean squares algorithm for generating an impulse response estimate that is useful for generating an echo cancellation signal to be subtracted from the echo containing signal.